1. Field of the Invention
This invention relates to the bonding of cellular glass bodies and more particularly to a bonded cellular glass body system and mortar therefor wherein the mortar comprises a substantial portion of a glass powder having a composition substantially comparable to that of the cellular glass being bonded and low alkali metal content.
2. Description of the Prior Art
Acid-resistant mortars and coatings for insulating systems are well known in the art. Such mortars are generally employed in bonding insulating systems which are exposed to cyclic acidic environments. For example, in an industrial smoke stack, as in a coal-burning power plant, a brick lining has been employed which brick lining was bonded with an acid-resistant silicate-containing mortar. The mortar and brick were acid resistant in order to avoid damage due principally to the presence of sulfuric acid formed by hydrolysis of sulfur oxides in the stack gases.
The early acid-resistant mortars comprised wet mixtures of silica sand and sodium silicate which were "cured" by evaporation and exposure to acid, either as a wash, or while in service. Later mortars were developed that enabled more rapid construction. These contained hydrolyzable compounds such as sodium silicofluoride that reacted with the silicates causing gelation. Other setting agents such as hydrolyzable organic compounds were developed that gave longer pot life. Potassium silicate also was used in place of sodium silicate to prevent formation of hydrated sodium salts and their resultant destructive expansion.
Mortars with much lower alkali metal ion content were formulated with sodium silicate powders as setting agents. U.S. Pat. No. 2,244,325 discloses a basic sol which is formed by passing a dilute alkali silicate solution through a suitable ion-exchange medium, such as zeolite, whereby the alkali metal ions are removed to yield a colloidal solution, or sol, of silica. All the foregoing mortars were developed to bond dense acid-resistant brick systems and did perform very well. Brick densities were in the order of 140 pounds per cubic foot which insured sufficient pressure on the mortar joint to promote a good bond. Moreover, the linear coefficients of thermal expansion of brick and prior art mortars were comparable (ca. 7-9 .times. 10.sup.-.sup.6 in/in/.degree.F) so as to minimize spalling and thermal shock failures. The bricks were not adversely affected by the alkali metal ions.
More recently however, superior insulation compositions have been developed, particularly in the area of cellular glass type compositions. For example, cellular boro-silicate glass compositions in the form of blocks or slabs have been developed which are acid resistant and which have excellent insulating properties. The cellular silicates however, differ substantially from bricks in both physical and thermal characteristics. The cellular boro-silicate glasses have a density of only about 9- 15 pounds per cubic foot and while being an imporatnt advantage from one standpoint, that density does not provide sufficient joint pressure to provide a good bond with prior art mortars. Moreover, a cellular boro-silicate glass composition may typically have a linear coefficient of thermal expansion of about 1.6 .times. 10.sup.-.sup.6 in/in/.degree.F, far lower than that of bricks or prior art mortars. The low coefficient is again desirable from an application standpoint, but causes bond failures with existing mortars. Additionally the surface of cellular glasses is sensitive to the alkali metal ion content.
Accordingly, there is a need for acid resistant compositions useful in bonding cellular glass bodies such as bodies of cellular boro silicate compositions. Moreover, there is a need for mortar of the type described that is able to provide a bonded system having good thermal shock characteristics. Additionally, there is a need for acid resistant mortar compositions having thermal shock characteristics compatible with those of cellular glass bodies so as to readily provide an efficient bonded glass body system useful in an acid environment.
This invention provides acid resistant mortar compositions and cellular glass body systems bonded therewith which overcome the disadvantages of known mortars and systems, and will be further understood as the specification proceeds.